Construction joint position in suspended flat slab

[JacksPanic]

thread507-276229

With reference to thread507-276229 , why can a CJ not be placed at midspan? Surely that is where the bottom part of the concrete will form a crack anyway? Top part will just push together? Anybody feel like a debate?

 

[hokie66]

No debate from me. Midspan is where I want construction joints, except in prestressed slabs, in which case you want them where the strands are at middepth.

 

[irishengdave]

At a construction joint you need to splice the bottom bars together. If they screw it up on site and the lap is not enough you have problems. You don't want that at the max sagging moment location. Try and put it close to the point of contraflexure ie 1/3 span.

 

[hokie66]

irish,
Bottom reinforcement normally is continuous through construction joints in slabs. There is rarely a reason to splice bars at construction joints. You put joints at or near midspan because the shear is minimal there.

 

[ron9876]

I agree with hokie. Middle 1/3 of span unless it is a PT slab.

 

[JAE]

Yes - my vote - middle 1/3 of the spans.

 

[irishengdave]

I have Never seen bottom bars or top bars continuous at a constr joint. I guess it depends on your area/state/province of practice. In my experience normal practice is 1/3 span. I am visualizing 12 foot long bars cantilevering from the edge of the slab getting in the contractors way now!

 

[JAE]

irishengdave - what typically happens on flat slab construction is that the full bars are set/chaired over the formwork deck and then construction joint bulkheads are installed across a line of rebar.

The fabricators many times do not know the sequence or staging of concrete placements and simply fabricate the rebar based on the design layout.

By staying in the middle 1/3 you minimize the cases where the CJ's will intersect top bars extending past the joint location.

 

[hokie66]

irishengdave,
JAE has described it correctly. The formwork and reinforcement are all in place, and the construction joint is just to limit the amount of concrete that has to be placed and finished at one time.

 

[irishengdave]

agreed if the joint is because of concrete supply. If the contractor wants to focus his efforts elsewhere and leave part of the slab incomplete then a splice may be necessary. Depends on the situation.

 

[hokie66]

It is not just because the contractor wants to focus elsewhere. There are practical limits to the amount of concrete that can be placed and finished in a day. Many contractors like to limit it to 1000 or 1200 square metres for a single crew, and this depends somewhat on the building's shape. If you are constructing a shopping centre or convention centre floor which is 50000 square metres, there must be construction joints, and these need to be planned. You can't let them just happen.